基于均匀化理论粘弹性复合材料松弛规律的研究

基于均匀化理论研究了粘弹性复合材料的松驰规律,推导出了均匀化方法计算等效松驰模量的一般形式。在Laplace域中计算等效松弛模量,并用最小二乘法拟合Prony级数表示的松弛函数,然后在时间域中进行数值的Laplace逆转换,得到时间域内等效复合模量。给出了算例来验证这个计算方法的可行性并讨论了松弛模量随夹杂体积分数变化的规律。     

On the shape of effective inclusion in the Maxwell homogenization scheme for anisotropic elastic composites

The paper focuses on the main uncertainty involved in classical Maxwell’s (1873) homogenization method for elastic composites. Maxwell’s scheme that equates the far fields produced by a set of inhomogeneities and by a fictitious domain with unknown effective properties (“effective inclusion”) is re-written in terms of the compliance and stiffness contribution tensors. It is shown that the shape of the effective inclusion substantially affects the overall elastic properties. The choice of this shape in the case of anisotropic composite is a non-trivial problem that has never been discussed in literature. In this paper, we show that the problem appears due to incompleteness of the Maxwell scheme and show that the problem can be realized when the effective inclusion is of ellipsoidal shape. We discuss how the aspect ratios of the ellipsoid have to be chosen and illustrates the approach by two examples – material with cracks having orientation scatter and a three-phase transversely-isotropic composite. It is also shown that tensor of the effective elastic constants calculated in the framework of Maxwell’s scheme is always symmetric with respect to couples of indices.     

高熔点差三元合金累积叠轧-扩散合金化制备工艺

为解决高熔点差多元合金制备方法存在的元素偏析、合金性能受限、制备成本高等问题,提出了高熔点差组元合金的累积叠轧-扩散合金化制备新工艺。采用SEM、EDS、TEM、XRD和万能试验机表征了累积叠轧-扩散合金化Cu-21Ni-5Sn合金的组织和性能,研究了累积叠轧和阶梯式扩散热处理工艺对Cu-21Ni-5Sn合金成分均匀性的影响和机理,并揭示了后续时效制度对Cu-21Ni-5Sn合金性能的影响和机理。结果表明：通过累积叠轧7道次+650 ℃/5 h+1000 ℃/8 h阶梯真空扩散热处理工艺,制备出了元素误差小于5%、成分均匀的Cu-21Ni-5Sn合金。采用累积叠轧实现减薄中间层、缩短扩散距离,增加晶界、位错等原子扩散通道,低熔点Sn元素与Cu、Ni元素在650 ℃形成高熔点(Cu,Ni)₃Sn金属间化合物临界层,在1000 ℃高温加速Cu、Ni元素扩散。Cu-21Ni-5Sn合金在40%预冷变形下于470 ℃时效60 min充分调幅分解,基体中析出致密的与基体共格的DO22及L12有序固溶体,与α铜基体之间的取向关系为(-1-1-1 )Cu//(-2-20)DO22,(-200)Cu//(-310)L12。合金抗拉强度达到峰值916 MPa,弹性模量为135.4 GPa,合金导电率达到6.23% IACS。     

ZK60-1Nd镁合金的组织结构及腐蚀行为

通过扫描电镜(SEM),X射线衍射仪(XRD),电化学工作站和盐水浸泡失重法研究了ZK60-1Nd镁合金的组织结构及在3.5%(质量分数)NaCl水溶液中的腐蚀行为。结果表明:ZK60-1Nd镁合金主要由基体α-Mg相,沿晶界分布的含有稀土元素Nd的粗大网状相组成。ZK60-1Nd镁合金在3.5%的NaCl水溶液中的腐蚀速率为0.2mg·cm~(-2)·h~(-1);经过400℃保温96h后,部分共晶相熔断成尺寸约为0.5μm的颗粒,腐蚀速率降低到0.1mg·cm~(-2)·h~(-1);经过500℃保温8.0h,网状共晶相熔化成分散块状,基体中细小棒状相溶解消失,Nd,Zn和Zr元素在基体中含量增加且分布均匀,腐蚀速率达到最低,为0.03mg·cm~(-2)·h~(-1)。其耐蚀性能改善的主要原因为α-Mg基体中固溶态Nd元素含量的增加导致合金自腐蚀电位提高以及基体α-Mg中腐蚀微电偶数目的减少。     

Effect of homogenization treatment on the fracture behaviour of silicon nitride/graphene nanoplatelets composites

Si₃N₄ based composites with 7 wt.% of graphene nanoplatelets (GNPs) were prepared using different homogenization methods. Si₃N₄/GNPs powder mixtures were dispersed in isopropanol and homogenized by attritor milling, ball milling or planetary ball milling. The ball milling technique was also used for the homogenization of Si₃N₄/GNPs mixture in dry state. Fractography analysis was carried out in order to assess the individual homogenization treatment. Depending on the homogenization methods, the size of the processing flaws varied from 20 μm up to 400 μm. The agglomeration of the GNPs and the residual porosity were found as the most frequently observed types of the critical flaws. The planetary ball milling with previous ultrasonication of GNPs in isopropanol was found to be the most promising homogenization technique, resulting in the composites with the highest bending strength (average value is 740 MPa) and the lowest average size of the processing flaws (around 20 μm).     

Nonlinear homogenization techniques to solve masonry structures problems

The behaviour of masonry material subjected to different in-plane loading combination is studied in this work. The masonry is considered as a periodic composite material composed by a regular distribution of brick and mortar and it is analyzed using a homogenization technique. The mechanical properties of the masonry, as an orthotropic homogeneous material, depend on the geometrical and mechanical properties of the components based on the study of the equilibrium and compatibility of a basic cell. The masonry is a frictional material and its behaviour depends on the loading direction, for these reasons, a unilateral damage model is chosen for the analysis. This model describes the behaviour of brittle materials subjected to tension-compression cyclic loads based on the introduction of two damage variables and it assumes that the damage is due to the beginning and growth of cracks only in the mortar joints. It is considered that the bricks have a linear elastic constitutive relationship. Numerical applications are performed with a nonlinear finite element code in order to test the proposed procedure by comparing the results with those available in the literature and also with experimental data.     

Numerical derivation of homogenized dynamic masonry material properties with strain rate effects

Masonry is a composite material composed of bricks and mortar disposed in a regular arrangement. It is commonly used as load bearing or partition walls in building structures. Owing to limitations of computer power, detailed distinctive modelling of brick and mortar of a realistic masonry structure or a structure with masonry infilled walls is usually not possible. Moreover, no dynamic masonry material model can be found in the open literature. Dynamic masonry material properties are important for an accurate prediction of masonry failure and fragmentation under dynamic loads. In this paper, a continuum damage model with strain rate effect is developed for masonry materials based on the homogenization method. The equivalent elastic properties, strength envelope and dynamic increase factors (DIFs) of strength and moduli for the homogenized masonry material are numerically derived from the simulated responses of a representative volume element (RVE). A numerical model of an RVE is analyzed with detailed distinctive modelling of brick and mortar with their respective dynamic material properties obtained from laboratory tests. The homogenized material model can be used to analyse large-scale masonry structures subjected to dynamic loading.     

New methods for estimating the torsional rigidity of composite bars

We obtain general bounds on the torsional rigidity and give examples on how these bounds can be used to obtain estimates and simple formulae which yield more accurate information than the classical treatment. Moreover, we use the homogenization theory to estimate the limit of the torsional rigidity when the characteristic length of the composite microstructures decreases.     

Acoustical and optical branches of wave propagation in random particulate composites

The work is dedicated to the analysis of acoustical and optical branches of longitudinal elastic wave propagation in the medium with a random set of spherical inclusions. The effective field method and quasicrystalline approximation are used for the construction of the dispersion equation for the wave number of the mean (coherent) wave field propagating in the composite. This dispersion equation serves for all frequencies of the incident field, properties and volume concentrations of inclusions. Different branches of the solutions of this equation are obtained and analyzed. Each of these branches may be interpreted as a specific mode of wave propagation, and its input in the mean (coherent) wave field is essential only in a certain frequency region. The predictions of the method are compared with some experimental data existing in the literature.